This invention relates, in general, to regulators, and more particularly, to switching power supplies.
Motor protection circuitry is found on many types of motors for detecting failures and to deactivate the motor should a failure occur. One common type of motor which incorporates motor protection circuitry is a three phase motor. Motor protection circuitry for a three phase motor typically consists of circuitry to monitor the magnitude of current in the individual windings. The current must be within a predetermined range for the motor to be operating within specification. If the current of a winding is outside the predetermined range it could indicate a failure in the motor winding.
Initially, the motor protection circuitry used was electro-mechanical in nature. The electro-mechanical systems were assembled from discrete components. Motor manufacturers have pursued the development of solid state motor protection circuits to reduce cost and manufacturing complexity. The problem with solid state motor protection circuits is that they require some source of supply voltage to power the circuitry.
A battery may be used for a power supply, but this requires the user of the motor to periodically replace the battery. A separate power supply for powering the motor protection circuitry is not cost effective. An approach with merit is to derive the power for operating the circuitry from the motor windings itself. The current in the motor windings can be monitored by inductively coupling to windings with current transformers. The current transformer output is modeled as a controlled current source which is dependent on the loading of the motor. For example, a heavily loaded motor requires more current through the windings which also increases current from the current transformer. Conversely, a lightly loaded motor requires less current through the windings which decreases current from the current transformer. This current could also be used to develop a power supply voltage for the motor protection circuitry.
One method used to generate a regulated voltage source from an inductively generated current (from the motor windings) is to charge a capacitor with the current. A zener diode in parallel with the capacitor limits the voltage on the capacitor. The zener diode breaks down above a predetermined voltage. Forming a regulated voltage using this technique is undesirable for two reasons: first, the power dissipation of the zener diode is high at large input currents and second, the regulation of the voltage is poor over the wide range of motor windings available. The regulation can be poor at high currents due to the internal resistance of the zener diode.
It would be of great benefit if a voltage regulator circuit could be developed that provides a regulated voltage from an input current that operates over a wide variety of input currents and reduces power dissipation.